Related art involves the light source taking several minutes to come up to the proper functioning intensity. For a given current when the light source is excited, the light source reveals an intensity that is much greater than desired for the application at hand, such as a fiber optic gyro. For the fiber optic gyro to be functioning as soon as possible, two to three minute delay is unacceptable. A satisfactory warm-up time is two to three seconds.
Conventional applications of a laser diode use automatic power control or automatic current control to regulate the light output of the diode. Automatic power control typically incorporates a back facet diode (in the same package as the laser diode) to provide a signal proportional to the output intensity of the laser diode. That signal is used to servo the signal intensity (by varying the laser diode current) to a desired level. However, automatic power control has the disadvantage of producing a change in wavelength in the output light as the current varies. That change in wavelength can cause unacceptable errors in a device such as a fiber optic gyro (FOG).
Automatic current control uses a current source to provide an essentially constant current through the laser diode. Automatic current control provides constant wavelength as long as the temperature of the diode remains constant. The disadvantage of the automatic current control is that the efficiency of a laser diode is very nonlinear with temperature. The current levels required at normal operating temperatures produce an excessive light intensity signal at low temperatures. Typically the devices are operated at some controlled temperature to minimize the intensity and wavelength variations. Conventional temperature controllers use Peltier elements to cool the devices to some temperature below the normal ambient operating temperature.
A problem with any temperature controlled laser diode circuit is achieving a usable signal while the device operating temperature is stabilizing immediately after turn-on. The efficiency of the diode (defined here as the intensity produced for a given amount current applied) is inversely proportional to the temperature of the diode. That means a laser diode operating at low temperatures but with the current required at normal temperatures will produce more intensity than the associated electronics circuitry can accommodate. Some means of reducing the current while the device is warming up, is required if the device is to function during that interval.